In the past, substantial efforts have been devoted to the investigation and development of more accurate and less complex flow meters for measuring fluid flow in a variety of circumstances. The accuracy of a flow meter refers to the extent within which the actual flow parameters may differ from the meter reading. Repeatability refers to the ability of a given meter to provide consistent readings over a large number of repetitions. In many situations, the repeatability of a flow meter is more important than its accuracy. For example, in a large industrial process where constant conditions are maintained to provide maximum efficiency, the repeatability of a flow meter may be extremely important.
In recent years, it has become increasingly important to provide a visual readout in conjunction with many types of flow meters. A readout may be accomplished by means of a mechanical arrangement in which a dial or gauge is adjusted in response to changes in flow, or electrical signals may be generated to provide a remote readout through an electrical display or other suitable means. In many situations, a readout capability is considered mandatory, while in other situations, it is generally advantageous.
Despite the advantage provided by a remote readout capability, the additional instrumentation and electronics necessary to provide such a capability have made it uneconomical to provide such a capability for many flow meters used in low cost applications. Such components as signal preamplifiers, pulse rate converters, pulse rate indicators, and range extenders and linearizers greatly add to the cost of a flow meter and often make a visual readout capability economically unfeasible.
In a prior application, Ser. No. 266,622, filed May 22, 1981, of which I am a co-inventor, several varieties of flow meters are described. The flow meters of that prior application, which application is specifically incorporated herein by reference, all have the capability of providing a remote readout. In addition, the flow meters described and claimed in that prior application are extremely accurate and provide an increased measure of repeatability in comparison with prior art flow meters. These advantages are achieved in the invention of application Ser. No. 266,622 by utilizing a magnet or magnetic field producing element in the flow meter, and a Hall effect transducer for sensing the position of the magnetic element in the device. The displacement of the magnet is responsive to the rate of flow or change in pressure of the fluid being measured. Thus, a change in the relative position of the magnet with respect to the Hall effect sensor changes the magnetic flux density sensed by the Hall effect device. The output voltage of the Hall effect sensor varies in accordance with the flux density in the vicinity of the Hall effect sensor. This, the movement of the magnet in response to the rate of flow of the fluid through the flow meter causes a proportional change in the output voltage of the Hall effect sensor. The position of the magnetic element is determined by the difference in pressure on the upstream and downstream surfaces of the magnetic element.
While the flow meters described and claimed in application Ser. No. 266,622 solve many of the problems of the prior art, there are still certain drawbacks to the use of the meters shown as specific embodiments in that application. One of the primary disadvantages is a requirement that the output of the Hall effect sensor must be linearized in order to more correctly represent fluid flow. Linearity refers to a straight line function, while a non-linear relationship may be exponential or logarithmic. Obviously, the closer the function is to being linear, the more accurate will be the representation of the desired function. Although non-linear signals can be linearized electronically or pneumatically, this greatly adds to the cost of producing the device, and may adversely affect accuracy.
In addition to the non-linear nature of the output of the devices described in application Ser. No. 266,622, it has been found that the arrangement of the "slot" on one side of the housing in the specific embodiments described therein produces an unequal distribution of forces on the magnetic element in the flow meter. As a result of this unequal distribution, the magnetic element may encounter frictional and other forces which can reduce the accuracy of measurement of the device. In addition, the relatively large clearance between the magnetic element and the cylinder in which it moves allows fluid to flow between the magnetic element and the cylinder thereby bypassing the slot, and not being measured.
Accordingly, it is a primary object of this invention to improve the accuracy and repeatability of flow meters utilizing a Hall effect sensor.
It is a further object of this invention to economically provide a linearized output from a flow meter which is a direct function of the rate of flow of the fluid through the meter.
Yet another object of the invention is to balance the forces on a magnetic element in a flow meter for more accurate measurement by an accompanying Hall effect device.
A still further object of the invention is to prevent leakage around the magnetic element of a flow meter having a Hall effect device, thereby increasing the accuracy of measurement of the device.